Organogallium compounds, particularly trialkyl gallium compounds, have found increasing use in the semiconductor industry. In this industry a gallium compound is deposited onto a suitable substrate, generally together with one or more compounds of a Group 5 element, such as arsenic or phosphorus. The deposition of such compounds is carried out via the decomposition of organometallic compounds from the vapor phase. Such decomposition is known as Metal Organic Chemical Vapor Deposition (MOCVD). When epitaxial layers are grown from such decomposition the technique is better known as Metal Organic Vapor Phase Epitaxy (MOVPE).
A convenient route for the preparation of trialkyl gallium compounds is via the reaction of gallium (III) chloride with either a Grignard reagent, i.e., an alkyl magnesium halide, or an alkyl lithium compound. A disadvantage of these methods resides in the use of gallium (III) chloride which is difficult to obtain in the purity that is required for products used in the semiconductor industry. High purity gallium is available commercially and is therefore a suitable starting material for the preparation of trialkyl gallium compounds.
It is known to prepare trialkyl gallium compounds from alloys or mixtures of gallium and magnesium by reaction with alkyl halides according to the following reaction: EQU 2Ga+3Mg+6RX.fwdarw.2R.sub.3 Ga+3MgX.sub.x,
in which R represents an alkyl group and X is halide. Such reactions not only require an excess of magnesium, but also a super-stoichiometric amount of alkyl halide. All this adds to the cost of the trialkyl gallium production. When a sub-stoichiometric amount of magnesium is used one would expect that the yield of trialkyl gallium compounds, based on original gallium, would decrease. Unreacted gallium would be present in the reaction product which would represent a considerable disadvantage because high purity gallium is expensive.
In U.K. patent specification No. 2,123,423 a process for the preparation of trimethylgallium or triethylgallium is described in which an alloy Ga.sub.2 Mg.sub.5 is reacted with methyl iodide in the presence of an ether. The ether is a relatively volatile ether, such as diethyl ether, or an ether with a relatively high boiling point, e.g., di-isopentyl ether or diphenyl ether.
In Soviet Union author's certificate No. 325,847, trialkyl gallium compounds are prepared from stoichiometric mixtures or alloys of gallium and magnesium by reaction with an alkyl halide. The yields of trialkyl gallium compounds were up to 65%. It has now been found that the yield of trialkyl gallium is substantially increased if the Ga--Mg alloy used contains a particular excess of magnesium.